Microwave antenna



Nov. 10, 1953 H. GRUENBERG 2,659,005

MICROWAVE ANTENNA Filed March 21, 1951 f 2 Sheets-Sheet 1 JWE/WUP HARRY GRUENBEPG- .BY-

ATTORNEYS- Patented Nov. 10, 1953 MICROWAVE ANTENNA Harry Gruenberg, Ottawa, Ontario, Canada, as-

signor to National Research Council, Ottawa, Ontario, Canada, a corporation of Canada Application March 21, 1951, Serial No. 216,718

1 Claim.

The invention relates to directive antennas for microwaves, that is for electro-magnetic waves having lengths in free space of less than one meter.

In copending United States application Serial Number 626,738 filed November 5, 1945, by W. H. Watson and E. W. Guptill for Directive Antennas for Micro-Waves (Patent Number 2,573,746 granted November 6, 1951), (see also a paper entitled Resonant Slots by W. H. Watson appearing on pages 747 to 777 of Volume 93, Part IIIA, Number 4 of the Journal of the In 'stitution of Electrical Engineers, and the book The Physical Principles of Wave Guide Transmission and Antenna Systems by W. H. Watson, 1947, Oxford at the Clarendon Press) there is disclosed an antenna structure comprising a waveguide formed with an array of radiators consisting of slots in special arrangements. The present invention is concerned with the arrangement in which adjacent slots are located on opposite sides of the centre line of a broad face of the waveguide and in the present application, an antenna with this arrangement of slots will be called the Watson and Guptill antenna. The Watson and Guptill antennatakes into account the well-known design considerations for linear arrays of radiators and the fact that the wave length in the guide differs from that in free space. For many purposes, for example radar, it is essential that there should be only one effective beam from the array, and it is also desirable that this beam should be substantially normal to the axis of the array, that is normal to a straight line passing through all the radiators. In order that the beam should be normal to the axis of the array, the radiators must radiate in phase; This could be achieved by spacing the radiators along the axis of the array at intervals equal to the wavelength in the guide (to) of the wave to be radiated, but, owing to the higher phase velocity in the guide, at least in the usual case where the dielectricis air, such a spacing would, in a guide having a normal width of about of the wavelength in free space (As), be equal to the spacing of about 11/2)\S. Suchan array would be unsatisfactory because its radiation pattern would contain second-order beams of substantially the same intensity as the main beam making angles of approximately 45 degrees with the latter (see pages 101 to 107, Aerials for. Centimetre Wavelengths by Fry and Goward published in 1950 by the Cambridge University Press). The beam could also be made normal to the axis of the array by coupling adjacent radiators in reversed phase relation, that is, in such a way that if the adjacent radiators were situated in the same transverse plane in the guide, they would be excited by currents or voltages out of phase with each other, and by spacing the radiators at intervals of /zxg. A spacing of xg is, in the normal case of a guide of a Width of about %)\s, equivalent to a spacing of about %As, and is consequently satisfactory from the point of view of the radiation pattern; When the spacing of the radiators coupled in reverse phase relation is different from kg; the angle of the beam to the axis of the. array becomes acute and decreases with increasing departure from \g spacing, the beam being directed in the direction of wave propagation in the guide when the spacing is greater than 180 and in the opposite direction when the spacing is less than 180.

The present invention resulted from the dis-, covery that even when the above conditions are observed in the design of a slotted waveguide antenna such as the Watson and Guptill antenna, there remain second order beams making angles of approximately 50 with the main beam and having levels of about 10 per cent of that of the main beam. In many types of radar work, this spurious radiation is not troublesome, for example, in the case of navigation on open wa-, ters where the radar equipment is not ordinarily used to locate nearby objects. However, in the case of navigation on narrow waters, for example in a canal or a river, many nearby objects may be encountered, such as other boats or the banks of the canal or river, and, if second order beams cause reflections from these nearby objects, serious errors in interpretation of the radar patterns may result. In some cases the second order beams may causethe patterns to be blurred and unreadable. It is thought that the reason these second order beams have remained undiscovered, prior to the present invention, is that, as mentioned above, they are not noticeable except in the presence of nearby objects and'moreover they occur in planes in which radiation patterns are not usually taken. Al-. though it was previously, appreciated that a di rective antenna obtained by spacing the slots about a guide wavelength on one side of the. centre line and exciting them approximately in phasewould result in large second order beams due to the slot spacing being greater than a free space wavelength, it was generally thought that this defect was overcome and at the same time the directive ieature of the array was retained by spacing the slots approximately half a guide wavelength apart and reversing the phase of alternate slots by staggering them about the centre line. This reasoning was based on the theory of linear arrays and tacitly assumed that the same displacement of the slot radiators from a straight line has negligible effect on the radiation pattern. By the discovery which led to the present invention, it was found that this assumption, in general, is not justified, since such an array actually has second order beams even for slot spacings of half a guide wavelength.

In copending United States application Serial No. 216,717 filed on even date with the present application, for Microwave Antennas, by the present applicant, there is disclosed an antenna structure which greatly reduces the second order beams. This structure has the longitudinal centre lines of the slots (in a broad face of a waveguide) collinear and the narrow faces of the waveguide shaped so that the centre line between the narrow faces is sinuous and lies on op posite sides of adjacent slots. Although the performance of this structure is highly satisfactory with regard to reduction of the second order beams, its construction is more expensive than would be desired for some applications. An antenna according to the present invention is cheaper in construction but just as effective in reducing the second order beams.

According to the present invention :a microwave antenna structure of the slotted waveguide type has a pair of substantially parallel plates extending from the broad face which contains the slots. The plates are parallel to the centre line of the broad face and are spaced one on either side of the series of slots. The distance between the plates is less than a half wavelength in free space, and the length of the plates in a direction normal to the broad face is at least substantially a wavelength in free space. Preferably the parallel plates have diverging plates fixed along their outer edges.

An antenna according to the present invention costs little more than the prior art slotted wave- Shown inFigure 3,94 Series of Slots H in the broad guide antenna, but has the advantage of providing substantial suppression of the second order beams which are radiated by the prior antenna and which are of disadvantage in radar Work over short distances, for example, navigation in 1 narrow waters.

The invention will be further described with reference to the accompanying drawings which illustrate certain embodiments of it, and in which Figure 1 is a perspective diagrammatic view of a slotted waveguide microwave antenna according to the prior art,

Figure 2 is a perspective representation o'fse'c- 0nd order beams produced by a prior art antenna as shown in Figure 1,

Figure 3 is a perspective view of a section 'of a slotted waveguide microwave antenna according to the present invention, and

Figure 4 is typical radiation patterns for a prior art antenna and for an antenna according to the present invention.

A prior Watson and Guptill slotted wave guide antenna is shown in Figure 1, where a slotted rectangular waveguide It has longitudinal slots II in one of its broad faces l2. Adjacent slots II are staggered about the centre line l3 of the broad face l2. As explained in the Watson and Guptill copending application, as well as in the paper and book by W H. Watson, referred to above, the dimensions of the slots "H should be such that they are resonant for the wavelength of the microwave to be radiated and are of substantially greater length than width. It is also explained in the references mentioned that the distance of the slots H from the centre line l3 controls the amount of radiation from the slots II. The usual X, Y and Z axes are shown in Figure 1, and vectors [4 and I5 respectively at angle 9 from the Z axis and at angle 4 from the X axis define the usual polar and azimuthal angles in a spherical co-ordinate system.

The prior art antenna of Figure 1 is shown in dotted line in Figure 2 with the second order beams t6 and 1! shown in full line. The second order beams 16 and I! have, respectively, broad maxima 18, I9 and 20, 2| of which the axial planes are symmetrically located about the axial plane through the main beam, making angles of approximately 50 with the latter. The maxima are at (0 and as indicated in Figure 1) where As and to are the free-space and guide wavelengths, respectively, and x is the deviation of the slot spacing from 1r radians.

The maximum values expressed "as fractions of the peak of the main beam are approximately:

where '6 is a weighted mean slot offset. This weighted mean is close to the amount of offset of the slot which is most strongly excited.

For a typical waveguide array these second order beams l6 and I1 may be ten per cent of the main beam.

A slotted waveguide antenna according to the present invention is shown in Figure 3.

face 12 of the waveguide are arranged about the centre line 13 in accordance with the teachings of the prior art. In accordance with the present invention .a pair of parallel plates 30 extend from the broad face 12 in parallel relation to the centre line [3. The plates :30 are arranged one on either side of the series of slots H and have diverging plates 3] fixed along their outer edges. The plates 31 may be integral with the plates 30. In accordance with the present invention the plates 30 are fixed apart .a distance b of .less than a half of the wavelength in free space of the microwave to beradiated andhave lengths -c in a direction normal to the broad face .12 .of at least a wavelength in free space of ithemicrowave to be radiated. The plates '30 may be fixed to the broad face "[2 of the waveguide by any of the well known methods, for example, sweating,

I by which a good electrical connection between from the point at which the waveguide is to be fed with a microwave.

Figure 4 shows typical radiation patterns for a prior art slotted waveguide type of antenna and for a slotted Waveguide type of antenna according to the present invention. Each of the graphs is relative field strength plotted against the angle 0 (see Figure 1) and each of the graphs for the same array is for a different angle (see Figure 1). It is clear, from a comparison of the field strengths of the two arrays for the same angle that with an array according to the invention the undesirable second order beams 16 and I! are greatly reduced although the main beam 28 is practically unchanged.

What I claim as my invention is:

A directive antenna for microwaves comprising a waveguide of substantially rectangular cross-section with two opposite narrow faces and two relatively broad faces, said broad faces having a width of substantially three-fourths of a wavelength in free space, the waveguide having formed in a broad face thereof a series of slots substantially resonant for a Wave of a given wavelength and of substantially greater length than width, the centres of the slots of the series being spaced along the length of a linear section of the waveguide at intervals of less than substantially seven-eighths of said wavelength in free space, each of the slots of the series being formed with its length parallel to the longitudinal centre line of said broad face, adjacent slots lying on opposite sides of said centre line, the distance from said centre line of substantially every slot being at least substantially the same as the distance from said centre line of either adjacent slot, a pair of flat conducting plates arranged in substantially parallel relation and extending from said broad face throughout the length of said linear section, said plates being substantially parallel to said centre line and spaced one on either side of the series of slots, the distance between said plates being less than a half of said Wavelength in free space and the length of said plates in a direction normal to said broad face being at least substantially said wavelength in free space, whereby said plates provide flat parallel conducting surfaces bounding the space in front of said slots and being spaced apart a distance greater than the width of a slot.

HARRY GRUENBERG.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,433,368 Johnson et a1 Dec. 30, 1947 2,470,016 Clapp May 10, 1949 2,501,105 Steinberger Mar. 21, 1950 2,572,628 Kock Oct. 23, 1951 2,573,746 Watson et a1. Nov. 6, 1951 2,574,433 Clapp Nov. 6, 1951 

